EGF is the prototypic member of a large family of polypeptide growth factors that are known to exert numerous physiological actions in the gut. In the stomach, EGF has been shown to stimulate cellular growth and proliferation and to regulate the expression of the H,K-ATPase gene, a marker of parietal cell differentiation. The intracellular signal transduction pathways that mediate the multiple, complex actions of growth factors in the stomach have been only partially elucidated. We have recently observed that incubation of canine parietal cells in primary culture with EGF for 16-18 h results in induction of H,K-ATPase gene expression through a signal transduction pathway that requires the activation of protein kinase B/Akt. In contrast, prolonged exposure (>72h) of the parietal cells to EGF appears to induce dramatic morphological changes which are associated with inhibition of H+/K+-ATPase gene expression and stimulation of parietal cell proliferation through a MAPK dependent mechanism. On the basis of these observations we hypothesize that the duration and the intensity of the signals generated by activation of the EGF receptor are responsible for the induction of specific programs of parietal cell activation. Thus, while a short exposure of the parietal cells to growth factors results in the expression of a highly differentiated cellular phenotype, prolonged stimulation of the cells with EGF, leads to the activation of mechanisms that induce parietal cell de-differentiation. Accordingly, the overall goal of this application will be to dissect the molecular mechanisms that regulate parietal cell differentiation/de-differentiation. In the first specific aim we will investigate the effect of incubation of the parietal cells with EGF for different time periods on H,K-ATPase gene expression and gastric acid production. We will examine if prolonged stimulation of the parietal cells with EGF leads to increase TGF-a production and TGF-a peptide release. We will also determine by the use of specific immunohistochemical markers, if EGF induces trans-differentiation of the parietal cells into mucus cells or fibroblasts. In the second specific aim will examine the intracellular signal transduction pathways that mediate parietal cell differentiation/de-differentiation in response to growth factor stimulation by focusing our efforts on the role of the MAPK and Akt kinase signal transduction pathways. Finally in the third specific aim, we will investigate the role of the Akt signal transduction pathway "in vivo" by studying genetically engineered mice overexpressing either constitutively or dominant negative forms of Akt kinase in the gastric parietal cells. Through the proposed studies, we hope to shed new insights that will have implications not only in the broad scientific areas of physiology and cellular and molecular biology, but also in the arena of clinical medicine since we believe that our studies may contribute to a better understanding of the pathophysiological mechanisms that lead to the development of both metaplastic and neoplastic lesions in the stomach.